The present invention relates, in general, to sewing machines and, more particularly, to a new and useful method and device for the formation of an overcast seam by means of a zigzag sewing machine.
U.S. Pat. No. 4,250,824 discloses a method for forming an overcast seam by means of zigzag sewing machine which provides that, after a first intertwining of needle thread and looper thread, in a first overstitch position of the needle bar the needle bar alternate into a second overstitch position to form a second intertwining of the thread, the needle thread being laid over an oversew mandrel and thereby slackened. In the second overstitch position of the needle bar, at least one straight stitch is then formed, whereupon the needle thread, leading from the first to the second thread loop, is pulled out to form a loop, in the next following alternation of the needle bar into the first overstitch position, is offered to the needle for stitching and then released. The alternation of the needle bar into the respective other overstitch position takes place while the material feed is interrupted. To obtain a particularly loose seam, the piece of thread between the first and second thread loop is pulled out while the material feed is reversed. The progressive seam formation is achieved exclusivly by executing at least one straight stitch within a basic overcast seam formation of at least four stitches.
Due to feeding intermissions or the execution of a reverse stitch, the progressive seam formation suffers from a significant delay so that the effective feed rate and, hence, the sewing time are adversely affected. This method can be economically executed only on zigzag machines equipped with controls for the needle bar and the fabric feeder.
The oversew mandrel, in the device for the implementation of the known method, serving to slacken and pull out the needle thread leading from the first to the second thread loop of a basic overcast seam formation, is of hookshaped design at its free end to prevent slip off of the thread to be pulled. However, when stiff or slippery threads are used, this is not a sufficient safety measure to insure against slipping off. While such slipping off could be prevented, to a great extent, by opening the hook up more, this would cause stripping problems for the pulled-out thread loop because, for stripping, the oversew mandrel of the known device is moved past and caused to make contact with a loop stripper hoop fastened to the front edge of the fabic presser of the sewing machine. If a more widely opened oversew mandrel were used for better security against the thread loop slipping off, the thread in the known device would be unduly stressed when stripping the thread loop.